Osmoregulation (College Board AP® Biology)
Study Guide
Written by: Phil
Reviewed by: Lára Marie McIvor
Osmoregulation & Homeostasis
The importance of Transporting Molecules Across Membranes
Growth
The growth of an organism requires transport of molecules across the cell membrane to support metabolic processes. provide necessary nutrients and eliminate waste products
Cell membranes are responsible for the regulated movement of ions, nutrients and other molecules across the cell membrane
This ensures the maintenance of homeostatic mechanisms which are responsible for the growth of the organism
Homeostasis
Homeostasis is the process of maintaining constant internal body conditions
These specialist control systems are critically important in ensuring optimal conditions for enzyme action, cell function and growth
Organisms employ various homeostatic control systems, detecting internal and external conditions through sensory cells and activating processes to keep them within the correct range
Examples of physiological factors that are controlled by homeostasis in mammals include:
Core body temperature
Metabolic waste (eg. carbon dioxide and urea)
Blood pH
Concentration of glucose in the blood
Optimum water content
Concentration of the respiratory gases (carbon dioxide and oxygen) in the blood
Each of these mechanisms relies on the constant movement of molecules across the membranes of cells
e.g. movement of glucose out of the blood when blood glucose levels rise too high, or reabsorption of water from the nephron in the kidney if the water potential of the blood becomes too low
Osmoregulation and Homeostasis
Osmoregulation is the collective name for the range of methods used by organisms to maintain their optimum water content
Water is crucial to life due to its properties such as :
Water is a universal solvent
Water has a high specific heat capacity and heat of vaporization
(Water is covered in more detail in 1.1 Structure of Water & Hydrogen Bonding)
Osmoregulation allows organisms to control their internal water potential and solute composition
Some examples of functions that rely on osmoregulation include:
Regulation of blood water concentration by the kidneys of mammals
Cellular function - structure and metabolic functions of a cell depend on water content inside and outside
Biochemical reactions requiring water or specific concentrations of ions and other solutes
Excretion of waste products
Blood pressure regulation
Respiratory function - influenced by the moist lining of the respiratory tract
Temperature regulation
Reproductive processes e.g. development of embryos in aquatic organisms
Maintenance of Solute Potential
Organisms use osmoregulatory mechanisms to control their internal solute levels, and hence, water potential
The solute potential (Ψs) of a solution can be calculated in the following equation:
Ψs = -iCRT
where:
i = ionization constant (for a given solute)
C = concentration of the solute (in moles per liter)
R = pressure constant (0.00831 kJ mol-1 K-1)
T = temperature in Kelvin (°C + 273)
Worked Example
Agricultural land in low lying coastal areas is sometimes at risk of flooding from storm surges that bring saline seawater onto the land.
A farmer whose land suffered a recent saline flooding episode wished to estimate the impact upon his ability to produce crops.
Salinity levels (concentration of NaCl) were measured after the flood had receded at 200 mM (mmol per liter)
The ionization constant of NaCl is 2
R = 0.00831 kJ mol-1 K-1
The ambient temperature of the soil water was measured as 12 °C
(a) Calculate the solute potential of the soil water post-flood. You may assume that the salt is fully ionized in the soil water.
(b) How might this change of solute potential affect the farmer's crop?
(c) What measures could the farmer take to alleviate any problems caused by the flood?
Answers
(a) Use the equation
Ψs = -iCRT
where:
i = ionization constant (for a given solute)
C = concentration of the solute (in moles per liter)
R = pressure constant (0.00831 kJ mol-1 K-1)
T = temperature in Kelvin (°C + 273)
Step 1: convert units where necessary
Concentration of 200 nM needs to be divided by 1000 to give concentration in M; 200 ÷ 1000 = 0.2
Degrees C needs to be converted to Kelvin by adding 273; 12 + 273 = 285
Step 2: plug values into the equation:
Ψs = -2 × 0.2 × 0.00831 × 285
Ψs = -0.951 (MPa)
(b) This is a relatively highly negative number (versus pure water's solute potential of zero) so this makes the soil water hypertonic to the crop's root cells. This will draw water out of the root cells by osmosis, causing osmotic shock to the crop plants. The farmer is unlikely to be able to grow crops successfully on this land until it has its salt level lowered.
(c) Covering the soil with fresh water will leach out some of the salt from the contaminated soil. This is called 'ponding' or 'leaching'. Some farmers may switch to more salt tolerant species of crops, although this limits their production capability. Alternatively, the only practical solution is to wait for heavy rain to leach out the salt naturally.
Examiner Tips and Tricks
You'll always be given the relevant formulas in the Exam, although you should always be careful to convert all values to the appropriate units before performing the calculation in the given equation.
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